The invention relates to an optical modulation device comprising two substrates, which are provided with at least one electrode, and an optically active layer which is located between said two substrates, the optically active layer comprising a liquid crystalline material having ferroelectric, antiferroelectric or electroclinic properties. The invention also relates to a modulation device in the form of a display device. Said modulation devices can be used in electro-optic light shutters, for example for optical printers, and in drivable optical filters and splitters.
An optical modulation device of the type mentioned in the opening paragraph is known per se from, for example, European Patent Appliction EP-A 406705. The display device or display described therein comprises two parallel transparent glass substrates which are provided with a matrix of transparent electrodes of, for example, indium-tin-oxide (ITO) on the sides facing each other. An orientation layer of, for example, rubbed polyimide is provided on said electrodes. The substrates are positioned with respect to each other by means of spacers, for example, in the form of optical fibres having a specific diameter. An optically active layer of liquid crystalline material is present between both substrates.
At the operating temperature of the device the liquid crystalline material is in the so-called chiral smectic C-phase (Sc*-phase). In this state the material exhibits ferroelectric properties such as spontaneous polarization. If the optically active layer is sufficiently thin all chiral liquid crystalline molecules are oriented by the orientation layers in such a way that they extend parallel to the substrate surfaces. Under said conditions the direction of polarization of the liquid crystalline molecules is substantially transverse to the substrate surfaces. Via the electrodes, an electric field can be applied transversely across the optically active layer. As a result, all chiral liquid crystalline molecules are polarized in the same direction. The direction of polarization can be reversed by means of an oppositely directed field. In the absence of an external magnetic or electric field an adjusted direction of polarization is maintained for a relatively long period of time. Thus, such a ferroelectric modulator exhibits bistability.
Polarizers having mutually crossed directions of polarization can be provided on the surfaces of the substrates facing away from each other. Such a device can be used as an electro-optic display device, in which the optical transparency of the various pixels can be switched by means of an electric field.
Such ferroelectric modulation devices have important advantages over the more widely known optical modulation devices comprising chiral liquid crystalline materials of the so-called TN-type (twisted nematic). The operation of the latter devices is based on changes in the orientation of the molecules of the liquid crystalline material in the nematic phase as a result of the application of an electric field. Due to the relatively long relaxation time of the liquid crystalline molecules after the field has been discontinued, such modulation devices have an intrinsically relatively long response time of the order of tens of milliseconds. In contrast, the operation of ferroelectric modulation devices is based on changes of the polarization direction of the molecules of the liquid crystalline material in the chiral smectic phase as a result of the application of an electric field. Said changes are intrinsically relatively fast. Consequently, ferroelectric modulation devices have a typical response time of a few microseconds. A further advantage of ferroelectric modulators is that, in comparison with TN modulators, the image contrast is governed less by the viewing angle.
However, state-of-the-an ferroelectric optical modulation devices have a number of important disadvantages. One such disadvantage relates to providing and maintaining grey levels. It is known per se that such grey levels can be obtained by rapid modulation of the applied electric field. However, when this field is removed the adjusted grey level disappears soon. Thus, the use of the known modulators does not allow adjusted grey levels to be "passively" maintained for a relatively long period of time. Further, it is known from "Ferroelectrics", vol. 121, pp. 1-29 (1991) that by means of specific texturing processes grey levels can be obtained which, in the absence of an electric or magnetic field, are maintained for a relatively long period of time. In practice it has been found however that such texturing processes are very expensive and difficult to realise, while the reproducibility of the grey levels obtained leaves much to be desired. A further important disadvantage of the known ferroelectric modulators is their sensitivity to shocks, which was found to be relatively high. It is an object of the invention to overcome the above-mentioned disadvantages. The invention more particularly aims at providing an optical modulation device of relatively simple construction, which enables grey levels to be realised which are maintained in the absence of an electric field. In addition, the modulation device in accordance with the invention must have a relatively low sensitivity to shocks.